In a specimen-analyzing instrument, such as an x-ray microanalyzer (or EPMA: Electron Probe X-ray Micro-Analyzer), a number of imaginary object elements are defined in a two-dimensional plane on the surface of a specimen. An electron beam is directed to these object elements successively. Characteristic x-rays are produced by chemical elements .THETA..sub.1, .eta..sub.2, . . . .THETA..sub.n (n.gtoreq.2) at the positions of the object elements (x, y), and the intensities of the x-rays are detected. Data about these intensities is stored in a memory, and then data regarding the intensities of the characteristic x-rays emanating from an arbitrary chemical element .THETA..sub.i is read from the memory, in accordance with the operator's instructions. Finally, the data read out in this way is sent to a display device or recorder.
When the instrument is so operated that the intensities of the characteristic x-rays derived from the certain chemical element .THETA..sub.i is digitized into n-state (n.gtoreq.2) variables to obtain a color image on the principle of mapping, it is possible to roughly know the distribution of x-ray intensities. In providing such a color image, the signal indicating the intensities of x-rays is compared with plural reference levels to effect the aforementioned digitization. Then, different colors are assigned to values of different states, for creating a color image. Therefore, it is impossible to know the degree of homogeneity of the concentration of one element of interest within a region of the same color. That is, it is not possible to know whether the x-ray intensities at the object elements within this region lie in a wide or narrow range. The mapping techniques for obtaining color images are disclosed in "Microbeam Analysis 1985", p. 145-p. 147.
If it is possible to know the average weight concentration of an arbitrary chemical element .THETA..sub.j within a region of an arbitrary size at an arbitrary location on the viewing screen while a color image created by mapping is being observed, then more knowledge can be gained from this region. A known method of finding the weight concentration of an arbitrary chemical element is disclosed in "Microbeam Analysis 1985", p. 82-p. 84 and "Microbeam Analysis 1986", p. 271-p. 278.